Blogging On Earth: Humans acidifying ocean at unprecedented rate

by Sara E. Pratt
Thursday, March 1, 2012

Human emissions of carbon dioxide are currently acidifying the oceans at a rate unprecedented in the last 300 million years — since well before the dinosaurs evolved — according to a study published today in Science. More acidic water can dissolve the shells of many marine organisms, including reef- and shell-building species, such as clams, oysters and corals, as well tiny organisms that form the base of the food chain.

An international team of about 20 researchers, led by geochemist Bärbel Hönisch at Columbia University’s Lamont-Doherty Earth Observatory, conducted a literature review to compile a record of ocean pH levels extending back to the Permian, the longest acidification timeline put together to date.

The goal of the study was to identify rapid pulses of carbon in the geologic record — and their associated ocean effects such as acidification and anoxia — which might help researchers predict how the oceans will react to the current anthropogenic pulse of carbon dioxide. The researchers concluded, however, that no parallel may exist for the current or future situation.

“Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry — a consequence of the unprecedented rapidity of carbon dioxide release currently taking place,” the authors reported.

Since the dawn of the industrial revolution, the oceans have absorbed more than half of the carbon dioxide emitted by humans. Over the last 100 years, atmospheric carbon dioxide concentrations have increased 30 percent, to 393 parts per million, while ocean pH has dropped (become more acidic) by 0.1 units, to a pH of 8.1.

That is an acidification rate 10 times faster than the next-fastest event, which occurred 56 million years ago at the Paleocene-Eocene Thermal Maximum (PETM), when atmospheric carbon dioxide concentrations doubled with a disastrous effect. Over a period of about 5,000 years, average global temperatures increased by 6 degrees Celsius and the oceans underwent a 0.45 unit drop in pH. At the PETM, half of all benthic foraminifera, a group of shell-forming single-celled organisms that live on the seafloor, went extinct.

The Intergovernmental Panel on Climate Change predicts that ocean pH may fall another 0.3 units by the end of the century, to a pH of 7.8. If so, the researchers report, we may experience previously unseen ocean conditions.

“The current rate of (mainly fossil fuel) carbon dioxide release stands out as capable of driving a combination and magnitude of ocean geochemical changes potentially unparalleled in at least the last 300 million years of Earth history, raising the possibility that we are entering an unknown territory of marine ecosystem change,” the authors wrote.

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